1. Field of the Invention
The present invention relates to an Ag alloy film. More particularly, it relates to an Ag alloy film preferably used as a reflective film or a semi-transmissive reflective film for an optical information recording medium having high thermal conductivity/high reflectance/high durability in the field of optical information recording media, an electromagnetic-shielding film excellent in Ag aggregation resistance, or an optical reflective film on the back of a reflection type liquid crystal display device or the like. Further, the present invention relates to a sputtering target for use in deposition of such an Ag alloy film.
2. Description of the Related Art
For the reflective films or the semi-transmissive reflective films included in optical information recording media (optical disks), Au, Al, Ag, or alloys containing these as main components have been widely used from the viewpoints of thermal conductivity, reflectance, and durability.
The Ag-based reflective film containing Ag as a main component has the following features: it has high reflectance with respect to a violet laser for use in a next-generation optical disk, and high thermal conductivity required of a write-once/writable disk; and, in addition, the raw material cost thereof is lower as compared with an Au-based reflective film. Therefore, it is a promising material as a reflective film or a semi-transmissive reflective film. However, it is superior to an Al-based reflective film in terms of durability, but it does not have durability as high as that of the Au-based reflective film. In order to put it into actual use as a reflective film or a semi-transmissive reflective film of an optical disk, it is necessary to improve the durability without impairing the high reflectance and the high thermal conductivity inherently possessed by Ag.
As for the means for improving the durability of such an Ag-based reflective film, the following improvement measures have been reported. For example, the durability (chemical stability) is respectively improved by adding Au, Pd, Cu, Rh, Ru, Os, Ir, and Pt to Ag in U.S. Pat. No. 6,007,889, or by adding Pd and Cu to Ag in U.S. Pat. No. 5,948,497. Further, the present inventors also have proposed a method in which the durability (thermal stability in the inhibition of grain growth, or the like) is improved by adding rare earth metal elements to Ag in JP-A 15464/2002.
However, for high-speed recording DVDs or next-generation optical disks, the levels of characteristics required of the reflective film have been further raised. This results in demands for durability, thermal conductivity, and reflectance of higher levels than ever before.
Particularly, for the durability, there is a demand for high corrosion resistance against halogen elements including chlorine. This demand is particularly prominent for a write-once optical disk in which a halogen element-containing organic dye recording film, a protective film, an adhesive layer, and the like are directly stacked on a reflective film. Further, as distinct from a DVD, the next-generation optical disk is in an inverted stacked configuration obtained in the following manner. First, a reflective film is deposited on a transparent plastic substrate, and dielectric protective film/recording film/dielectric protective film/are stacked and deposited thereon. For this reason, the surface roughness of the reflective film must be extremely reduced in order to suppress the deterioration of the recording and reproduction characteristics. Further, the next-generation optical disk is required to be capable of keeping the stability of the surface roughness even when put under a thermal load.
Whereas, as for the thermal conductivity, the heat generated in the very small region of the recording film through laser light irradiation is required to be rapidly diffused. Thus, in order for the reflective film to also have the function as a thermal diffusion film, the film is required to have high thermal conductivity.
Further, as for the reflectance, the reflective film is required to have high reflectance also with respect to the violet laser for use in a high-speed DVD or a next-generation optical disk.
However, no Ag base alloy has been yet found which satisfies all these requirements. In order that the reflective film may ensure high reliability as being used for a high-speed DVD or a next-generation optical disk, there is a strong demand for an Ag base alloy which has all the required characteristics of high thermal conductivity, high reflectance, and high durability.
On the other hand, conventionally, an Ag film has found various uses because of its high visible light transmittance and excellent infrared shielding property. For example, an infrared-shielding Ag film transparent member obtained by forming Ag on a transparent substrate of glass or the like through sputtering or the like is used in order to improve the heating and cooling efficiency in a room. Further, since the Ag film is also excellent in radio wave shielding property, for example, in order to protect electronic equipments which may undergo mis-operation due to radio wave from an external radio wave, or in order to suppress the emission of the radio wave generated from electronic equipments, it is used in the following manner. An Ag film is applied as described above on the window pane of the laboratory in which the equipments are set; or an Ag film or an Ag film-applied substrate is mounted internally in or externally on each of the equipments.
However, the Ag film has low abrasion resistance, and further, it has insufficient durability against environment. Therefore, it will be deteriorated due to moisture or the like, and hence it is difficult to use for a long period. For this reason, a means of increasing the thickness of the Ag film has been adopted. However, a sufficient solution has not yet been made from the viewpoints of abrasion resistance and durability improvements. Eventually, the Ag film will be deteriorated with the passage of time, so that the pure Ag film lacks the practicality. Incidentally, through the increase in film thickness, the electromagnetic shielding characteristics (infrared shielding property and radio wave shielding property) are improved. However, the visible light transmittance is decreased, so that it becomes dark in the room.
Under such circumstances, as a technique for increasing the transmittance within a visible light region, and further, improving the abrasion resistance and the weather resistance of the Ag film, there is proposed a technique of coating the Ag film with a transparent dielectric film made of an oxide such as tin oxide, zinc oxide, or titanium oxide, or a nitride such as silicon nitride. Further, in order to improve the adhesion between the Ag film and the oxide or the nitride, there is also proposed a technique of inserting Cr or a Ni—Cr alloy layer between the Ag film and the oxide or the nitride.
In accordance with the technique, it is possible to reduce the optical reflectance of the Ag film. This produces the effects of allowing the reduction of the glaring feeling due to a reflected light from the Ag film, and providing a longer life period than that of the pure Ag film. However, even if Ag is coated with a transparent dielectric film, Ag aggregates from the defective portions such as pinholes and scratches of the transparent dielectric film itself as the starting points when exposed to air. As a result, the Ag film tends to undergo film breaking (i.e., break in the continuity of the film), so that film breaking occurs (the continuity of the film is broken) In such a case, the conductivity of the Ag film is lost, resulting in a remarkable reduction in electromagnetic shielding characteristics. Further, unlimited number of white points occur due to the aggregation on the Ag film-applied substrate surface of glass, film, or the like, resulting in reductions of designability and salability.
As techniques for improving the aggregation of such an Ag film, various techniques have been proposed. For example, in JP-A No. 315874/1995, there is proposed a heat ray-shielding glass obtained by forming a metal thin film prepared by adding at least one element selected from the group consisting of Pd, Pt, Sn, Zn, In, Cr, Ti, Si, Zr, Nb, and Ta in an amount of 5 to 20 mol % to Ag on the surface of a glass plate.
Whereas, in JP-A No. 293379/1996, there is proposed a technique for stacking a metal layer containing Ag as a main component, and Pd in an amount of 0.5 to 5 at % based on the amount of Ag, and transparent dielectric layers each containing one or more metal oxides selected from the group consisting of Zn, In, and Sn on a substrate in such a manner that the metal layer is sandwiched between the transparent dielectric layers.
Further, in JP-A No. 135096/1997, there is proposed an electromagnetic-shielding substrate obtained by adding one or more elements selected from the group consisting of Pb, Cu, Au, Ni, Zn, Cd, Mg, and Al in an amount of 3 at % to Ag. Whereas, in JP-A No. 231122/1999, there is disclosed a technique for attaining the improvement of the aggregation resistance of Ag by adding Pb, Cu, Au, Ni, Pd, Pt, Zn, Cd, Mg, and Al to Ag.
Still further, the present inventors have proposed a technique for attaining the improvement of the aggregation resistance of Ag by adding Sc, Y, and rare earth elements to Ag (JP-B No. 351572/2001).
Even with the proposals or the proposed Ag alloy films, the aggregation of Ag proceeds with passage of time, resulting in deterioration of the Ag alloy films. For this reason, for example, when the film is used with its surface coated with each of the Ag alloy films exposed to air, the aggregation of Ag occurs centering on the defective portions of the transparent film covering the Ag alloy film. Therefore, the result is that the film must be processed into a laminated glass or an insulating glass for use so that the Ag alloy film surface is not exposed to air, leading to an increase in manufacturing cost. Further, also in the case where the film is processed into a laminated glass or an insulating glass, white points occur unless it is processed into a laminated glass or an insulating glass immediately after Ag film formation. This results in a loss of the value in use as a commercially available product. Further, even in the case where it has been processed into a laminated glass or an insulating glass, the resulting glass does not have sufficient durability because long-term use results in deterioration of the Ag alloy film.
Incidentally, in recent years, a reflection type liquid crystal display device operating with a small power consumption because a lamp is not required to be included therein has received attention. An optical reflective film is essentially disposed as a reflector on the back of the reflection type liquid crystal display device. It reflects indoor light, natural light, or the like, and serves as a light source for image formation. For this reason, the higher the reflectance of the optical reflective film is, the brighter and the easier to see the formed image is.
Conventionally, a thin film of Al with a high reflectance has been used as the optical reflective film. However, in recent years, a thin film composed almost exclusively of Ag (Ag thin film) which has higher reflectance, and is also resistant to chemical corrosion has come into use as the optical reflective film.
However, in the case where the Ag thin film has been exposed into air for a long time under high temperatures during manufacturing of a liquid crystal display device, in the case where it has been exposed under high temperatures and high humidities for a long time during use after manufacturing, or in other cases, white turbidity and white points caused by the increase in size of crystal grains, the aggregation of Ag atoms, the oxidation of Ag, or the like occur, resulting in a decrease in reflectance. For this reason, it has not been possible to obtain a high reflectance inherent in Ag. Further, the inevitable heat history (to 200° C.) during device manufacturing causes the crystal grain growth and the aggregation of Ag atoms, which involve the increase in roughness of the thin film surface and the anomalous grain growth. This results in a difficulty in device formation, and a further reduction in reflectance.
Under such circumstances, a proposal has been made in which different kinds of elements are added to Ag for the purpose of preventing the growth of crystal grains of Ag and the aggregation of Ag atoms, and allowing high optical reflectance inherent in Ag to be exerted and kept.
For example, in JP-A No. 134300/1995, there is disclosed a thin film made of a metal which is more susceptible to oxidation than silver, specifically, a silver alloy (Ag base alloy) containing one, or two or more metals selected from the group consisting of magnesium, aluminum, titanium, zirconium, and hafnium.
Whereas, in JP-A No. 230806/1997, there is disclosed a thin film made of a silver-based metal material (Ag base alloy) which is an alloy with different kinds of elements for preventing the migration of silver element, specifically, one, or two or more kinds of metals selected from the group consisting of aluminum, copper, nickel, cadmium, gold, zinc, and magnesium.
However, even with the forgoing prior-art techniques, it has not been possible to sufficiently suppress the growth of crystal grains of Ag and the aggregation of Ag atoms. Accordingly, it has not been possible to ensure high optical reflectance inherent in Ag.